1. Field of the Invention
This invention relates to devices for vapor gas interrogation, and more particularly to chip-scale vapor cells.
2. Description of the Related Art
Advances in microelectromechanical systems (MEMS) have enabled a variety of miniaturized and chip-scale atomic devices used in, for example, gyroscopes, magnetometers and chip-scale atomic clocks. With reduced system dimensions come many advantages, including lower operating power and reduced manufacturing cost for the finished device. Of primary importance in many of these MEMS applications is an atomic vapor cell for use as a frequency-defining element, rather than traditional quartz-crystal resonators, for improved frequency stability.
As is typical for atomic vapor cells during their manufacture, the vapor cell is charged with a sample material that later produces an interrogation gas during heating and subsequent operation. Common sample material examples for atomic vapor cells include rubidium (Rb) and cesium (Cs). The vapor cell is permanently sealed after charging, often using anodic bonding between a silicon substrate containing an interrogation cell enclosing the sample material and a transparent window through which the gas is interrogated after heating. Various techniques have been developed for initially charging the miniaturized vapor cell, such as by transfer of the sample material into the vapor cell using a pin head, heated vapor dispensation or microdroplet dispensing. Of particular concern for any charging method, is the sample material's exposure to oxygen and water vapor. Such exposure produces oxide and hydroxide contaminants which may later result in obscuration of the transparent windows of the vapor cell. Additionally, anodic bonding of the silicon substrate to the glass windows may be frustrated by migration of the sample material itself to the bonding surface prior to or during charging and/or bonding, especially as such bonding surfaces are narrowed in an overall effort to miniaturize the devices.
A need continues to exist for improved vapor charging techniques and apparatuses as such vapor cells are reduced in size.